Transparent double-sided adhesive sheet for image display device and image display device using the same

ABSTRACT

A novel transparent double-sided adhesive sheet for image display device is provided, that, under the constraint of holding down the thickness of the adhesive sheet to 250 μm or less, can relieve a distortion arising within the adhesive sheet after laminating even if a member has a stepped portion of 50 μm to 100 μm in height on the laminating surface. 
     Proposed is a transparent double-sided adhesive sheet for image display device which is used for the purpose of laminating an image display device constitutive member having on a laminating surface a stepped portion of 50 μm to 100 μm in height and a flat surface portion, and another image display device constitutive member, wherein a thickness of the maximum thickness portion of the adhesive sheet is 250 μm or less, and a gel fraction (a) at a position in contact with a stepped portion after laminating is 10% or greater and smaller than a gel fraction (b) at a position in contact with a flat surface portion.

This application is a continuation of U.S. patent application Ser. No.14/371,564, filed on Jul. 10, 2014, which is a National Stage entryunder 35 USC 371 of PCT/JP2012/083919, filed on Dec. 27, 2012, andclaims priority to Japanese Patent Application No. 2012-007681, filed onJan. 18, 2012.

TECHNICAL FIELD

The present invention relates to a transparent double-sided adhesivesheet that can be used to laminate a constitutive member for displaydevice. In particular, it relates to a transparent double-sided adhesivesheet that can be used suitably in order to laminate a constitutivemember for display device having a stepped portion on the laminatingsurface, and to an image display device using the same.

TECHNICAL BACKGROUND

In recent years, in order to increase the visibility of image displaydevices, the gap between an image display panel such as a liquid crystaldisplay (LCD), a plasma display (PDP) or an electroluminescence display(ELD), and a protective panel or a touch panel member disposed on thefront side (viewing side) thereof, is filled with an adhesive sheet, aliquid bonding agent, or the like, to carry out suppression ofreflection of incoming light or outgoing light from a displayed image atthe air layer interface.

As a method that uses an adhesive for filling such a gap betweenconstitutive members for display device, a method is known, whereby aliquid adhesive resin composition containing a UV-curable resin isfilled in the gap and then illuminated with UV light to let it cure.However, such a method bears the problem that the operation when theliquid is filled being cumbersome, productivity is poor, and moreover,at locations that UV light has difficulty in reaching such as portionsmasked by a printed masking layer, curing the adhesive is difficult, andobtaining a stable quality is difficult.

In addition, the method of filling the gap between constitutive membersfor display device using an adhesive sheet is also known. For instance,as a transparent adhesive sheet that can be used suitably for laminatinga transparent panel such as a protective panel or a touch panel to animage display panel, a transparent adhesive sheet is described in PatentDocument 1, which is a adhesive sheet having at least one or more eachof a first adhesive layer and a second adhesive layer having differentviscoelastic behaviors and is provided with a constitution in whichthese layers are layered integrally, the value of the dynamic shearstorage modulus G′ as measured with a temperature distribution at 1 Hzfrequency being within a specific range.

A transparent double-sided adhesive sheet is described in PatentDocument 2, which is a transparent double-sided adhesive sheet having amiddle resin layer (A) and pressure-sensitive adhesive layers (B) as thefront and back side layers, each layer being respectively a layer withone or more species of (meth)acrylic acid ester series (co)polymer asthe base resin, the storage shear modulus (G′(A)) of the middle resinlayer (A) at 1 Hz frequency in the 0° C. to 100° C. temperature rangebeing higher than that of the pressure-sensitive adhesive layers (B),and, the indentation hardness (Asker C2 hardness) of the entirety of thesheet being 10 to 80.

In addition, as a thin (for instance, 30 to 50 μm-thick) adhesive sheetthat is applicable onto a surface having a step or a protrusion, aUV-crosslinking adhesive sheet is described in Patent Document 3, whichis a UV-crosslinking adhesive sheet comprising a (meth)acrylic copolymerof a monomer containing a (meth)acrylic acid ester having aUV-crosslinking site, the store elastic modulus of the adhesive sheetbefore UV-crosslinking being 5.0×10⁴ Pa or greater but 1.0×10⁶ Pa orlower at 30° C. and 1 Hz and 5.0×10⁴ Pa or lower at 80° C. and 1 Hz, andfurther, the store elastic modulus of the adhesive sheet afterUV-crosslinking being 1.0×10³ Pa or greater at 130° C. and 1 Hz.

PRIOR ART REFERENCES Patent Documents

-   [Patent Document 1] International publication brochure WO2010/044229-   [Patent Document 2] International publication brochure WO2011/129200-   [Patent Document 3] Japanese Patent Application Laid-open No.    2011-184582

SUMMARY OF THE INVENTION Problems to be Solved by the Invention

In the field of image display devices, centered around mobile phones,mobile terminals and the like, diversification of designs has beenproceeding in addition to decrease in thickness and increase inaccuracy, and new problems have been arising concomitantly to this. Forinstance, while printing a black masking portion in the shape of a frameat the periphery of a surface protective panel is common in prior art,concomitantly to diversification of designs, forming of thisframe-shaped masking portion in colors other than black has begun.

However, when forming a masking portion in a color other than black,since the masking ability is low with colors other than black, thethickness of the masking portion, that is to say, the printed portion,needs to be thick compared to when using black. Then, an adhesive sheetfor laminating a constitutive member provided with such a printedportion calls for a step-following ability, which, by following a largeprinted step, allows for filling throughout. In addition, the thicknessof the printed portion becoming large, there is the possibility that, atthe portion in contact with the printed portion, the image displaydevice becomes under large stress compared to other portions, whichgenerates distortion and negatively affects optical properties, and thussuppressing such a distortion is also sought.

Moreover, as the decrease in thickness of the image display device isproceeding increasingly, such a problem needs to be solved withouthindering a decrease in thickness.

Thus, the present invention relates to a transparent double-sidedadhesive sheet for image display device that is used in order tolaminate an image display device constitutive member having a steppedportion on the laminating surface, and provides a novel transparentdouble-sided adhesive sheet for image display device that, under theconstraint of holding down the thickness of the adhesive sheet to 250 μmor less, can relieve a distortion arising within the adhesive sheetafter laminating even if a member has a stepped portion of 50 μm to 100μm in height on the laminating surface.

Means to Solve the Problems

The present invention proposes a transparent adhesive sheet for imagedisplay device, which is a transparent adhesive sheet for image displaydevice that is used in order to laminate together an image displaydevice constitutive member having on the laminating surface a steppedportion with another image display device constitutive member,

the transparent adhesive sheet for image display device is formed by anon-solvent series adhesive composition which does not contain a solvent(also referred to as “a non-solvent series adhesive composition”),

the thickness of the thickest portion of the adhesive sheet being 250 μmor less, and

the gel fraction (a) at a position becoming in contact with the steppedportion when laminating being 10% or greater and smaller than the gelfraction (b) at a position becoming in contact with the flat surfaceportion when laminating.

In addition, in an image display device provided with at least twofacing constitutive members for display device, at least oneconstitutive member for display device having on the laminating surfacea stepped portion of 50 to 100 μm in height and a flat surface portionexcluding the stepped portion, the present invention proposes an imagedisplay device provided with a constitution comprising the abovetransparent double-sided adhesive sheet for image display deviceproposed by the present invention filled in between the two constitutivemembers for display device.

Effects of the Invention

According to the transparent double-sided adhesive sheet proposed by thepresent invention, even in a case of a member having a stepped portionof 50 μm to 100 μm in height on the laminating surface of theconstitutive member for display device to be laminated, by adjusting,under the constraint of holding down the thickness of the adhesive sheetto 250 μm or less, the gel fraction (a) at a position in contact withthe stepped portion after laminating to be smaller than the gel fraction(b) at a position in contact with the flat surface portion, distortionarising in the portion in contact with the stepped portion can berelieved, allowing negative effects on optical properties to besuppressed. Thus, the image display device and the transparentdouble-sided adhesive sheet for image display device proposed by thepresent invention can be used suitably in image display devices forwhich decrease in thickness and diversification in design are advancing,for instance, personal computers (PCs), mobile data terminals (PDAs),mobile terminals such as mobile phones, gaming machines, televisions(TVs), car navigation systems, liquid crystal pen tablets, furthermore,image display terminals with a touch panel function, and the like.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 Cross-sectional view showing an example of state after laminatingtwo constitutive members for display device using a transparentdouble-sided adhesive sheet for image display device according to oneexample of the present invention.

FIG. 2 Cross-sectional views showing an example of method for laminatingtwo constitutive members for display device using a transparentdouble-sided adhesive sheet for image display device according to oneexample of the present invention: (a) is an exploded cross-sectionalview showing an example of state before laminating and (b) is across-sectional view showing an example of state after laminating.

EMBODIMENT FOR CARRYING OUT THE INVENTION

Hereinafter, an example of embodiment of the present invention will bedescribed; however, the present invention is not limited to theembodiment below.

<The Present Adhesive Sheet>

The transparent double-sided adhesive sheet for image display deviceaccording to the present embodiment (hereafter referred to as “thepresent adhesive sheet”) is a adhesive sheet that is used in order tolaminate together a constitutive member for display device having on alaminating surface a stepped portion of 50 μm to 100 μm in height andanother constitutive member for display device integrally, thetransparent double-sided adhesive sheet being transparent and comprisingboth front and back sides as adhesive sides.

(Gel Fraction)

The present adhesive sheet is provided with a property that allows forletting the gel fraction after adhesion be different depending on thesite within the adhesive sheet.

For instance, it is possible, after a constitutive member for displaydevice having a stepped portion of 50 μm to 100 μm in height on thelaminating surface and another constitutive member for display devicehave been laminated together, for the gel fraction (a) at a position incontact with the stepped portion to be rendered smaller than the gelfraction (b) at a position in contact with the flat surface portion.

As a more concrete example, it is possible for the gel fraction (a) ofthe region B within the adhesive sheet in contact with the steppedportion to be rendered smaller than the gel fraction (b) of the region Ain contact with the flat surface portion, as shown in FIG. 1.

By rendering the gel fraction (a) at a position in contact with thestepped portion smaller than the gel fraction (b) at a position incontact with the flat surface portion in this way, the stress receivedby being compressed by the stepped portion can be relieved, allowing thedistortion arising in this portion to be rendered small, thus, thedifference in distortion at the position in contact with the steppedportion and the position in contact with the flat surface portion can berendered small, allowing negative effects on optical properties to besuppressed.

From such point of view, it is desirable for the difference [(b)−(a)]between the gel fraction (a) of the under-the-step portion and the gelfraction (b) of the under-the-flat-surface portion to be 5 to 30%, ofwhich 5% or greater or 25% or less, and, of this, 5% or greater or 20%or less in particular, is further desirable.

If the difference in gel fraction is 5% or greater, the stress receivedby being compressed by the stepped portion can be relieved, and thedifference in distortion at the position in contact with the steppedportion and the position in contact with the flat surface portion can berendered small. Meanwhile, if the difference in gel fraction is 30% orless, it prevents that, due to the difference in gel fraction within theadhesive sheet being too large, on the contrary, distortion arises,negatively affecting optical properties.

It is necessary for the gel fraction (a) of the position in contact withthe stepped portion after laminating to be 10% or greater, of which arange of 30 to 80% is desirable to be within; of this, to be 40% orgreater or 70% or less, whereof 50% or greater or 70% or less inparticular, is further desirable.

If the gel fraction (a) is 10% or greater, the stress of theunder-the-step portion can be relieved suitably while the shape of theadhesive sheet is being held to an extent that flowing does not occur,and if 30% or greater, a desirable adhesive force can be obtained.Meanwhile, if 70% or less, stress received by being compressed by thestepped can be relieved.

It is desirable for the gel fraction (b) of the position in contact withthe flat surface portion after laminating to be within a range of 35 to100%; of this, to be 45% or greater or 95% or less, whereof 55% orgreater or 90% or less in particular, is further desirable.

If the gel fraction (b) is 35% or greater, a desirable adhesive forcecan be obtained.

As methods for letting the gel fraction after adhesion be differentdepending on the site within the adhesive sheet as described above,forming methods such as those in the following can be cited.

(1) At a stage prior to laminating of constitutive members for displaydevice, an entire sheet of photocuring-type adhesive sheet iscrosslinked (curing: primary curing) beforehand to an extent that nodeformation or flow is provoked during storage while flexibility that issufficient to have high step-following ability is maintained,concretely, in such a way that the gel fraction is within a range of 10to 70%. Then, after laminating of the constitutive members for displaydevice, if only the portion other than that in contact with the steppedportion, that is to say, the portion in contact with the flat surfaceportion, is let to crosslink (curing: secondary curing), either byirradiating a light from the exterior of the masking portion having astep such as a print-stepped portion, or, by superimposing a maskingsheet so as to cover the stepped portion and irradiating light throughthe masking sheet, it is possible to render the gel fraction (a) at aposition in contact with the stepped portion smaller than the gelfraction (b) at a position in contact with the flat surface portion,within the adhesive sheet after laminating (forming method 1).

(2) In addition, even by having a adhesive sheet A, which is a portionthat enters in contact with a stepped portion, and a adhesive sheet B,which is a portion that enters in contact with a flat surface portion,prepared separately so that each is of the desired gel fraction, and,after integrating the two, laminating constitutive members for displaydevice together, it is also possible to render the gel fraction (a) at aposition in contact with the stepped portion to be smaller than the gelfraction (b) at a position in contact with the flat surface portion,within the adhesive sheet after laminating (forming method 2).

(3) Furthermore, having an entire sheet of photocuring-type adhesivesheet crosslinked (curing: primary curing) beforehand in such a way thatthe gel fraction is within the range of 10 to 70%, a masking sheet issuperimposed so as to cover the stepped portion and light is irradiatedthrough the masking sheet, or the like, such that only the portion otherthan that which is to be in contact with the stepped portion, that is tosay, the portion that is to be in contact with the flat surface portion,is let to crosslink (curing: secondary curing), to have the gel fraction(a) at a position that is to be in contact with the stepped portionrendered smaller than the gel fraction (b) at a position that is to bein contact with the flat surface portion, prior to laminating ofconstitutive members for display device. Thereafter, constitutivemembers for display device may be laminated together through theadhesive sheet prepared in this way (forming method 3).

Among the above methods 1 to 3, forming method 1 is the most desirablemethod.

That is to say, according to forming method 1, when applying to thelaminating of a constitutive member for display device having a steppedportion of 50 to 100 μm in height, by adjusting the gel fraction at thestage of primary curing by crosslinking to an extent that allows astepped portion to be followed sufficiently and, moreover, does notrender the sheet excessively flexible, sufficiently infiltratingthroughout by following the stepped portion and causing no air bubble orthe like to arise is possible, and, the stress due to the steppedportion can be relieved, furthermore, it is possible to prevent theproblems of bubbling or peeling at the laminating interface fromoccurring even if the portion in contact with the stepped portion isexposed to an environment of high temperature and high humidity orexposed to a sudden temperature change.

Further in addition, by performing secondary curing after laminating toraise the gel fraction of the flat surface portion and raise thecohesive force, that is to say, the adhesive force, laminating firmlytogether two constitutive members for display device is possible, andmanifesting excellent bubbling resistance is possible, as an entireadhesive sheet.

Forming method 1 will be described in further detail using figures.

First, the entirety of a adhesive sheet 2 is crosslinked (primarycuring) beforehand in such a way that the gel fraction is 10 to 70%.

Next, as shown in FIG. 2(a), mediated by the above-mentioned adhesivesheet 2, a constitutive member 1 for display device having aprint-stepped portion (1) of 50 to 100 μm in height and a constitutivemember 3 for display device are layered. At this stage, since theadhesive sheet 2 is soft to a suitable degree, the step can be followedsufficiently while maintaining storage stability.

Further, such light as UV is illuminated from the exterior of theconstitutive member 1 for display device (FIG. 2(b)). Then, since theprinted step (1) blocks the light, at the portion in contact with theprinted step (1), either the light does not arrive or the light thatarrives is noticeably limited, whereas, at the portion in contact withthe flat surface portion (3) where there is no printed step (1), thelight arrives sufficiently and the crosslinking reaction of this portionproceeds which can let secondary-curing, allowing peel-resistance andbubbling resistance to be realized.

<Adhesive Composition>

An example of constitutive material that can be used suitably to formthe present adhesive sheet (hereafter referred to as “the presentadhesive composition”) will be described. However, by all means, this isan example, and the composition is not limited to this.

As the present adhesive composition, the constitution or the likethereof is not limited in particular as long as it is one that allowsfor letting the gel fraction after adhesion be different depending onthe site within the adhesive sheet, as described above. However, inorder to realize the forming methods 1 to 3, and among these, theforming method 1, it is desirable for the composition to be aphotocuring-type adhesive composition. Desirable among these are thosecontaining a base polymer having a molecular weight of a predeterminedrange, a photo-crosslinking initiator and, as necessary, a crosslinkingagent.

(Base Polymer)

From the point of view of adhesiveness, transparency and weatherresistance, and the like, the base polymer of the present adhesivecomposition is preferably a (meth)acrylic acid ester series polymer(with the meaning of including copolymers, hereafter referred to as“acrylic acid ester series (co)polymer”).

The acrylic acid ester series (co)polymer serving as the base resin canbe prepared with physical properties such as the glass transitiontemperature (Tg) and the molecular weight adjusted suitably by selectingthe species, composition ratio, furthermore, the polymerizationconditions, and the like, of the acrylic monomer or the methacrylicmonomer used in order to polymerize this [(co)polymer].

As acrylic monomers constituting the acrylic acid ester (co)polymer, forinstance, 2-ethylhexyl acrylate, n-octyl acrylate, isooctyl acrylate,n-butyl acrylate, ethyl acrylate, and the like can be cited as the mainraw materials.

Aside from these, a (meth)acrylic monomer having various functionalgroups may be copolymerized with the acrylic monomer, according to suchpurposes as conferring cohesive force, conferring polarity, and thelike. As the (meth)acrylic monomer having functional groups, forinstance, methyl methacrylate, methyl acrylate, hydroxyethyl acrylate,acrylic acid, glycidyl acrylate, N-substituted acrylamide,acrylonitrile, methacrylonitrile, fluorine-containing alkyl acrylate,organo-siloxy group-containing acrylate, and the like, can be cited.

In addition, various vinyl monomers that are copolymerizable with theacrylic monomers and the methacrylic monomers above, such as, vinylacetate, alkylvinyl ether and hydroxy alkylvinyl ether, can also be usedsuitably in the polymerization.

It is desirable that the present adhesive composition is of anon-solvent series, that is to say, does not contain a solvent, is hotmelt formable, and at the primary cured stage, has suitable adhesiveforce, and, has flexibility that allows the unevenness or a foreignsubstance on the adherend surface to be followed.

Whether the molecular weight of the base polymer is too large or toosmall, film production by hot melting is not possible. In addition, ifthe molecular weight of the base polymer is too small, there is thepossibility that, at curing (crosslinking) prior to laminating, noadhesive force is exerted, or, being too soft, handleablity is poor;conversely, if the molecular weight is too large, becoming hard at thecuring (crosslinking) prior to laminating, there is the possibility ofnot being able to follow the unevenness or a foreign substance on theadherend surface.

Thus, from such point of view, it is desirable to use an acrylic acidester series (co)polymer in which the mass average molecular weight ofthe base polymer is 100,000 to 700,000, in particular 200,000 orgreater, or 600,000 or lower, of which, in particular 250,000 or greateror 500,000 or lower.

Among these, using an acrylic acid ester series (co-)polymer which massaverage molecular weight (MW)/number average molecular weight (MN) is 5to 10, of which 6 or greater or 9 or less, is more desirable. A largemass average molecular weight/number average molecular weight means thatthe molecular weight distribution is broad, and if this value is aslarge as on the order of 5 to 10, since the low molecular weightcomponents and the high molecular weight components respectivelycontribute to capabilities on par with their molecular weights, such asfluidity, wettability and cohesive force, there is a tendency forprocessability and adhesive capability to be better than those withnarrow (homogeneous) molecular weight distributions.

(Crosslinking Agent)

As crosslinking agents used when crosslinking an acrylic acid esterseries (co)polymer, for instance, multi-functional (meth)acrylateshaving two or more (meth)acryloyl groups are desirable.

From such point of view, the amount of crosslinking agent, with respectto 100 parts by mass of base polymer, is preferably 0 to 30 parts bymass, in particular 20 parts by mass or less, of which 10 parts by massor less, and among these in particular 5 parts by mass or less.

(Crosslinking Initiator)

As crosslinking initiator used in the present adhesive composition, anintermolecular hydrogen-abstracting photo-polymerizing initiator (alsoreferred to as “hydrogen-abstracting photoinitiator”) can be cited as aparticularly desirable example.

A photopolymerization initiator generates a radical by light irradiationand becomes the starting point of the polymerization reaction in thesystem. The radical generation mechanisms in the (meth)acrylic acidester and vinyl ester systems are divided broadly into two classes: theintramolecular cleavage type, in which radicals are generated bycleavage and decomposition of a single bond of the photopolymerizationinitiator per se, and the hydrogen-abstraction type, in which aphotoexcited initiator and a hydrogen donor in the system form anexcited complex, causing the hydrogen of the hydrogen donor to betransferred.

When generating radicals by light irradiation, intramolecular cleavagetype initiators decompose and become different compounds, and thus, onceexcited, no longer have functions as reaction initiators; in contrast,for the hydrogen-abstraction type, even once excited, those among theinitiators that have not reacted return to the ground state, and thusare re-usable as reaction initiators. Therefore, compared tophotopolymerization initiators of the intramolecular cleavage type,photopolymerization initiators of the hydrogen-abstraction type survivereadily as active species in the system even after the composition hasbeen primary cured by UV light. Consequently, use as reaction initiatorsis possible when further crosslinking (secondary curing) by irradiatingUV light after laminating. In addition, photopolymerization initiatorsof the hydrogen-abstraction type, compared to intramolecular cleavagetype, are also excellent on the point that, there being littledegradation products of low molecular weight, there is little generationof outgas and eluents of degradate origin.

As hydrogen-abstraction type photoinitiators, benzophenone seriescompounds such as, for instance, benzophenone, 4-methyl benzophenone,2,4,6-trimethyl benzophenone, 4-phenyl benzophenone, 4-hydroxybenzophenone, 4,4′-dimethylamino benzophenone, methyl o-benzoylbenzoateand dibenzosuberone can be cited.

Further, thioxanthone series compounds such as thioxanthone, 2-chlorothioxanthone, 2-methyl thioxanthone, 2-isopropyl thioxanthone and2,4-dimethyl thioxanthone, anthraquinone series compounds such as2-methyl anthraquinone, 2-ethyl anthraquinone, 2-tertbutyl anthraquinoneand 2-amino anthraquinone, and α-dicarbonyl series compounds, such asbenzyl and camphor quinone can be cited.

These can also be used as a mixed component comprising a combination oftwo or more species. However, there is no limitation to the substancesgiven above as photoinitiators of the hydrogen-abstraction type. Inaddition, photopolymerization initiators of the intramolecular cleavagetype may be used in combination in a variety of proportions.

The amount of photopolymerization initiator added is not limited inparticular, and in general is adjusted preferably in a proportion of 0.1to 10 parts by mass, in particular 0.2 parts by mass or more or 5 partsby mass or less, and among these, 0.5 parts by mass or more or 3 partsby mass or less, with respect to 100 parts by mass of base resin.However, this range may be exceeded in balancing with other elements.

(Other)

When crosslinking an acrylic acid ester series (co)polymer, additivesmay be added suitably, as necessary.

<Layered Constitution>

The present adhesive sheet may be a sheet comprising a single layer or amultilayered sheet comprising two or more layers that have been layered.

When the present adhesive sheet is to be a multilayered transparentdouble-sided adhesive sheet, that is to say, when forming a transparentdouble-sided adhesive sheet with a layered constitution provided with amiddle layer and outermost layers, it is desirable to form the outermostlayers from the present adhesive composition described above.

(Thickness)

From the point of view of not obstructing a decrease in thickness of theimage display device, it suffices for the thickness of the presentadhesive sheet that the thickness of the maximum thickness portion is250 μm or less. In other words, the present adhesive sheet may be asheet whereof the thickness is uniform or may be a non-uniform sheetwhereof the thickness is different in portions, and in the case of asheet whereof the thickness is non-uniform, it suffices that thethickness of the portion with the largest thickness is 250 μm or less.

The present adhesive sheet can fill a step of on the order of 100 μmmaximum.

<Image Display Device>

The present adhesive sheet can be used suitably in an image displaydevice having a constitutive member for display device provided with astepped portion.

For instance, in an image display device provided with at least twofacing constitutive members for display device, at least oneconstitutive member for display device having on the laminating surfacea stepped portion of 50 to 100 μm in height and a flat surface portion,and the gap formed together by the flat surface portions of the twofacing constitutive members for display device being 250 μm or less, animage display device can be constituted, provided with a constitutioncomprising the two constitutive members for display device laminatedthrough the adhesive sheet for image display device.

In so doing, as a constitutive member for display device having on thelaminating surface a stepped portion of 50 to 100 μm in height and aflat surface portion, for instance, a constitutive member for displaydevice such as a protective panel having a constitution in which aframe-shaped masking printed portion is formed in the periphery can becited. Further, a surface protective panel of the touch-on-lens type, inwhich a touch panel function is integrated, can also be used.

Meanwhile, as constitutive members for display device to be laminatedthereto, for instance, touch panels, image display panels, and the like,can be cited.

As the image display panels, in addition to liquid crystal panels havinga constitution in which a liquid crystal layer is sandwiched by glasssubstrates, those having a constitution in which a polarization film hasbeen layered on the viewing side of the liquid crystal panel in order toprevent a decrease in picture quality due to surface reflection, and thelike, can be cited.

Further, image display panels of the in-cell types with a touch sensorfunction built into the pixels of the liquid crystal, in addition toon-cell types with a touch panel built-in within the image displaypanel, can also be used.

<Explanation of Terms>

In general, “film” refers to a thin and flat product, of which thethickness is extremely small compared to the length and width, maximumthickness being arbitrarily limited, and provided in general in the formof a roll (Japanese Industrial Standard JIS K6900), and in general,“sheet”, by definition under JIS, refers to a product that is thin, ofwhich the thickness is small compared given the length and width.However, the boundary between a sheet and a film is not certain, andsince there is no need in the present invention to discriminate the twoin wording, in the present invention, “sheet” is deemed included evenwhen referring to “film”, and “film” is deemed included even whenreferring to “sheet”.

In the present invention, when the statement “X to Y” (X and Y are anynumbers) is made, unless expressly stated otherwise, along with themeaning of “X or greater but Y or less”, the meanings of “preferablylarger than X” and “preferably smaller than Y” are included.

In addition, when the statement “X or greater” (X is any number) ismade, unless expressly stated otherwise, the meaning of “preferablylarger than X” is included, and when the statement “Y or less” (Y is anynumber) is made, unless expressly stated otherwise, the meaning of“preferably smaller than Y” is included.

EXAMPLES

Hereafter, description will be given in further details by way ofexamples and comparative examples; however, the present invention is notto be limited by these.

<Production of Constitutive Members>

First, constitutive members for fabricating the adhesive sheets 1 to 5described below were produced in the following manner.

(Layered Sheet 1 for Middle Resin Layer Formation)

An acrylic acid ester copolymer A (Mw=440,000; Mn=62,000; Mw/Mn=8;theoretical Tg: −50° C.) was prepared, comprising 75 parts by mass of2-ethylhexyl acrylate (homopolymer Tg (glass transition point of apolymer comprising 2-ethylhexyl acrylate alone, which has beenpolymerized): −70° C.), 20 parts by mass of vinyl acetate (homopolymerTg: +32° C.), and 5 parts by mass of acrylic acid (homopolymer Tg: +106°C.), which were random-copolymerized.

Mixed to 1 kg of this acrylic acid ester copolymer A were 100 g of theUV-curing resin propoxylated pentaerythritol triacrylate (“ATM-4PL”,manufactured by Shin-Nakamura Chemical Co., Ltd.) as a crosslinkingagent and 15 g of 4-methyl benzophenone as a photopolymerizationinitiator to prepare a middle resin layer composition (A-1).

Onto a polyethylene terephthalate film (“NP75Z01”, a PET filmmanufactured by Panac Corporation; thickness: 75 μm), of which one sidewas treated to be releasable, the middle resin layer composition (A-1)was heat-melted and coated on this side with an applicator so as tobecome 110 μm in thickness and then laid over a polyethyleneterephthalate film (“E7006”, a PET film manufactured by Toyobo Co.,Ltd.; thickness: 38 μm), of which one side was treated to be releasable,so as to come in contact with this side, to produce a layered sheet 1for middle resin layer formation comprising PET film/UV-crosslinkingmiddle resin layer (A-1; thickness: 120 μm)/PET film.

(Layered Sheet 2 for Middle Resin Layer Formation)

A middle resin layer composition (A-2) was prepared similarly to themiddle resin layer composition (A-1), except that 200 g oftrimethylolpropane triacrylate as a crosslinking agent and 15 g of1-hydroxy-cyclohexyl phenyl ketone as a photopolymerization initiatorwere mixed.

Onto a polyethylene terephthalate film (“NP75Z01”, a PET filmmanufactured by Panac Corporation; thickness: 75 μm), of which one sidewas treated to be releasable, the middle resin layer composition (A-2)was heat-melted and coated on this side with an applicator so as tobecome 130 μm in thickness and then laid over a polyethyleneterephthalate film (“E7006”, a PET film manufactured by Toyobo Co.,Ltd.; thickness: 38 μm), of which one side was treated to be releasable,so as to come in contact with this side, to produce a layered sheet 2for middle resin layer formation comprising PET film/UV-crosslinkingmiddle resin layer (A-2; thickness: 130 μm)/PET film.

(Layered Sheet 1 for Adhesive Layer Formation)

To 1 kg of the acrylic acid ester copolymer A, 20 g of 4-methylbenzophenone was added and mixed as a photopolymerization initiator toprepare an adhesive composition (B-1).

Onto a polyethylene terephthalate film (“MRA75”, a PET film manufacturedby Mitsubishi Plastics; thickness: 75 μm), of which one side was treatedto be releasable, the adhesive composition (B-1) was heat-melted, coatedand formed into sheet-form in such a way that the thickness was 60 μmover this side and laid over a polyethylene terephthalate film (“E7006”,a PET film manufactured by Toyobo Co., Ltd.; thickness: 38 μm), of whichone side was treated to be releasable, so as to come in contact withthis side, to produce a layered sheet 1 for adhesive layer formationcomprising PET film/UV-crosslinking adhesive layer (B-1; thickness: 60μm)/PET film.

(Layered Sheet 1′ for Adhesive Layer Formation)

A layered sheet 1′ for adhesive layer formation comprising PETfilm/UV-crosslinking adhesive layer (B-1′; thickness: 60 μm)/PET filmwas produced similarly to the layered sheet 1 for adhesive layerformation except that the coating substrate was changed to polyethyleneterephthalate film (“MRF50”, a PET film manufactured by MitsubishiPlastics; thickness: 50 μm) of which one side was treated to bereleasable.

<Production of Adhesive Sheet 1>

The PET films on both sides of the middle resin layer (A-1) in thelayered sheet 1 for middle resin layer formation were sequentiallypeeled and removed while PET films on one side of the adhesive layers(B-1) and (B-1′) in the layered sheets 1 and 1′ for adhesive layerformation were peeled and the exposed adhesive faces were sequentiallylaminated onto both surfaces of (A-1) with a laminator to produce amultilayered adhesive sheet comprising (B-1)/(A-1)/(B-1′).

Through the polyethylene terephthalate films remaining on the surfacesof (B-1) and (B-1′), UV-light was irradiated with a high-pressuremercury lamp so that the integrated amount of light at 365 nm wavelengthwas 1,000 mJ/cm², and (B-1), (A-1) and (B-1′) were let to UV-crosslink,producing a pre-secondary curing transparent double-sided adhesive sheet1 with a uniform thickness (total thickness: 240 μm).

<Production of Adhesive Sheet 2>

Over a release-treated polyethylene terephthalate film (“MRA75”, a PETfilm manufactured by Mitsubishi Plastics; thickness: 75 μm), the middleresin layer composition (A-2) and the adhesive composition (B-1) wereco-extruded so as to comprise adhesive layer (B-1)/middle resin layer(A-2)/adhesive layer (B-1), coated and formed so as to comprise adhesivelayer (B-1)/middle resin layer (A-2)/adhesive layer (B-1)=60/40/60 μmand overlaid with a release-treated polyethylene terephthalate film(“MRA50”, a PET film manufactured by Mitsubishi Plastics; thickness: 50μm) to form a multilayered sheet comprising PETfilm/(B-1)/(A-2)/(B-1)/PET film.

From one polyethylene terephthalate film side, UV-light was irradiatedwith a high-pressure mercury lamp so that the integrated amount of lightat 365 nm wavelength was 1,000 mJ/cm², and (B-1), (A-2) and (B-1) werelet to UV-crosslink, producing a pre-secondary curing transparentdouble-sided adhesive sheet 2 with a uniform thickness (total thickness:160 μm).

<Production of Adhesive Sheet 3>

To 1 kg of the acrylic acid ester copolymer A, 50 g of nonanedioldiacrylate as a crosslinking agent and 10 g of 4-methyl benzophenone asa photopolymerization initiator were added and mixed to prepare anadhesive composition (B-3).

This adhesive composition was heat-melted, coated and formed into a filmwith an applicator so as to become 170 μm in thickness over arelease-treated polyethylene terephthalate film (“MRF75”, a PET filmmanufactured by Mitsubishi Plastics; thickness: 75 μm) and arelease-treated polyethylene terephthalate film (“MRA50”, a PET filmmanufactured by Mitsubishi Plastics; thickness: 50 μm) was overlaid toproduce a sheet comprising PET film/adhesive layer (B-3)/PET film.

From one polyethylene terephthalate film side, UV-light was irradiatedwith a high-pressure mercury lamp so that the integrated amount of lightat 365 nm wavelength was 800 mJ/cm², and (B-3) was let to UV-crosslink,producing a pre-secondary curing transparent double-sided adhesive sheet3 with a uniform thickness (total thickness: 170 μm).

<Production of Adhesive Sheet 4>

To 1 kg of the acrylic acid ester copolymer, 100 g of hydrogenated rosinester (“PINECRYSTAL KE604”, manufactured by Arakawa Chemical Industries,Ltd.) and 20 g of a mixture of 4-methyl benzophenone and 2,4,6-trimethylbenzophenone (“ESACURE TZT”, manufactured by Lamberti) as aphotopolymerization initiator were added and mixed to adjust an adhesivecomposition. This adhesive composition was coated and formed into a filmwith an applicator so as to become 150 μm-thick over a release-treatedpolyethylene terephthalate film (“MRA75”, a PET film manufactured byMitsubishi Plastics; thickness: 75 μm), and overlaid with arelease-treated polyethylene terephthalate film (“MRF50”, a PET filmmanufactured by Mitsubishi Plastics; thickness: 50 μm).

From one polyethylene terephthalate film side, UV-light was irradiatedwith a high-pressure mercury lamp so that the integrated amount of lightat 365 nm wavelength was 1,000 mJ/cm², and the adhesive composition waslet to UV-crosslink, producing a pre-secondary curing transparentdouble-sided adhesive sheet 4 with a uniform thickness (total thickness:150 μm).

<Production of Adhesive Sheet 5>

An acrylic acid ester copolymer (Mw=1,000,000, theoretical Tg: −61° C.)was prepared, comprising 48 parts by mass of 2-ethylhexyl acrylate, 50parts by mass of 2-methoxyethyl acrylate (homopolymer Tg: −50° C.) and 2parts by mass of 4-hydroxybutyl acrylate (homopolymer Tg: −80° C.),which have been random-copolymerized.

With respect to 1 kg of this acrylic acid ester copolymer, 0.2 parts bymass in solid contents of adduct-type hexamethylene diisocyanate(“DURANATE P301-75E”, manufactured by Asahi Kasei) was added as acrosslinking agent to produce an adhesive material composition.

Over a release-treated polyethylene terephthalate film (“MRA75”, a PETfilm manufactured by Mitsubishi Plastics; thickness: 75 μm), theadhesive composition was coated, formed and dried, then, overlaid with arelease-treated polyethylene terephthalate film (“MRF50”, a PET filmmanufactured by Mitsubishi Plastics; thickness: 50 μm), and cured forone week under the conditions of 25° C. in temperature and 50% humidityto be crosslinked, producing a transparent double-sided adhesive sheet 5with a uniform thickness (total thickness: 150 μm).

<Production of Adhesive Sheet 6>

Over a release-treated polyethylene terephthalate film (“MRA75”, a PETfilm manufactured by Mitsubishi Plastics; thickness: 75 μm), an acrylicacid ester copolymer (Mw=160,000) comprising 87 parts by mass of2-ethylhexyl acrylate, 12 parts by mass of acrylic acid and one part bymass of 4-acryloyloxy benzophenone, which were random-copolymerized, wascoated and formed so that the thickness was 150 μm, then, arelease-treated polyethylene terephthalate film (“MRF50”, a PET filmmanufactured by Mitsubishi Plastics; thickness: 50 μm) was overlaid toproduce a adhesive sheet 6 with a uniform thickness (total thickness:150 μm).

<Evaluation>

For the adhesive sheets 1 to 6 obtained as described above, thefollowing evaluations were carried out.

(Cut-Processability/Storage Stability Evaluation)

With the release films still layered, the adhesive sheets 1 to 6 werecut using a Thompson die-cutter into 100 sheets with a 55 mm×85 mmThompson blade. The shapes of the edges were observed immediately afterthe cutting and after storing 100 sheets of cut product for one weekunder the environment of 25° C. and 50% humidity.

Those for which glue flashes and crushing of the edges were observed inten or more sheets immediately after laminating or after storage wereevaluated as “cross (fail)”, and those for which there were no glueflashes and no crushing of the edges in ten or more sheets were assessedas “circle (pass)”.

(Print-Step-Following Ability Test)

At the periphery of a 60 mm×90 mm×thickness 0.5 mm soda lime glass, a 10mm-wide, 80 μm-thick white print (total light transmittance: 0%) wasapplied to produce an evaluation glass substrate having an 80 μm printedstep in the periphery. This evaluation glass substrate is a substitutefor an image display device constitutive member having on the laminatingsurface a stepped portion of 50 μm to 100 μm in height and a flatsurface portion.

Produced as a test adherend to be laminated onto this evaluation glasssubstrate was one in which a polarizer (“NWF-KDSEGHC-ST22”, manufacturedby Nitto Denko Corp.) serving as an image display device constitutivemember has been pre-laminated over the entire surface on one side of aglass plate (60×90 mm×t 0.5 mm).

On the adhesive sheets 1 to 6 that were cut in the processabilityevaluation, one release film was peeled and the exposed adhesive facewas adhered with a hand roller so as to cover the print-stepped portionof the glass substrate. Next, the remaining release film was peeled, anuntreated soda lime glass was press-laminated against the exposedadhesive side under reduced pressure (absolute pressure: 5 kPa) and thenfinish-bonded by performing an autoclave treatment (60° C.; 0.2 MPa; 20minutes) to produce a laminate for print-step-following abilityevaluation.

The laminates for print-step-following ability evaluation were leftalone for one day in a normal state (temperature: 23° C.; humidity:50%), then, the external appearance was visually observed, and those inwhich lifting or peeling of the adhesive sheet arose near a printed stepwere evaluated as “cross (fail)” and those without lifting or peelingwere evaluated as “circle (pass)”.

(Bubbling Resistance Test)

For the laminates for print-step-following ability evaluation producedsimilarly to the print-step-following ability test, through the printedglass substrate, UV-irradiation was performed on the adhesive sheets 1to 6 so that UV light at 365 nm reached 2,000 mJ/cm² in integratedamount of light to UV-crosslink (secondary curing) the adhesive sheet ofthe open portion on the inside enclosed by the printed portion,producing a bubbling resistance test sample.

Each sample was left alone for one day in a normal state (temperature:23° C.; humidity: 50%) then cured for 6 hours in a thermo-humidistat at85° C. in temperature and 25% humidity, and the external appearanceafter curing was observed visually.

Those in which new lifting or bubbling arose after curing were evaluatedas “cross (fail)” and those in which no new lifting or bubbling arosewere evaluated as “circle (pass)”.

(Gel Fraction)

For the laminates for print-step-following ability evaluation producedsimilarly to the print-step-following ability test, the glass substratewith the print and the glass with the polarizer were separated byimpregnating in liquid nitrogen to solidify by cooling the adhesivematerial (that is to say, the adhesive sheets 1 to 6). In contact withthe stepped portion and the flat surface portion of the glass substratewith the print, the adhesive materials (that is to say, the adhesivesheets 1 to 6) at the respective positions were collected to measure thegel fraction (a) of the under-the-step portion and the gel fraction (b)of the under-flat-surface portion by the following method.

Among the adhesive materials (that is to say, the adhesive sheets 1 to6) described above, portions of the locations in contact with theunder-the-step portion and the flat surface portion, each approximately0.05 g worth, were collected, wrapped into bag-form with an SUS mesh(#200), which mass (X) was measured beforehand, the mouth of the bag wasfolded shut, the mass (Y) of this bag was measured, then, after beingimmersed in 100 ml of ethyl acetate and stored in the dark at 23° C. for24 hours, the bag was taken out, heated at 70° C. for 4.5 hours toevaporate the attached ethyl acetate, the mass (Z) of the dried bag wasmeasured, and the determined masses were substituted into the equationbelow to determine the gel fraction.gel fraction [%]=[(Z−X)/(Y−X)]×100

TABLE 1 Adhesive sheet No. 1 2 3 4 5 6 Gel fraction (a) 63 59 74 10 66 0 Gel fraction (b) 80 82 81 71 66 65 Processability/storage ∘ ∘ ∘ ∘ ∘ xstability Unevenness-followability ∘ ∘ ∘ ∘ ∘ ∘ Bubbling resistance ∘ ∘ ∘∘ x ∘ reliability Overall evaluation ∘ ∘ ∘ ∘ x x<Discussion>

By adjusting the thickness of the adhesive sheet to be 250 μm or lessand the gel fraction (a) at a position in contact with the steppedportion after laminating to be 10% or greater and smaller than the gelfraction (b) at a position in contact with the flat surface portion, theadhesive sheets 1 to 4 were all found to be able to relieve a distortionarising in the portion in contact with the stepped portion, allowingnegative effects on optical properties to be suppressed.

In addition, according to the laminating methods that used the adhesivesheets 1 to 4, since the adhesive composition was primary-cured byUV-crosslinking at the stage of layering onto an image display deviceconstitutive member, there was sufficient flexibility for fillingunevenness, such that the printed steps could be followed. Then, afterlayering, by irradiating UV light through the printed portion (maskingportion), although the crosslinking of the portion in contact with theprinted portion (masking portion) did not proceed since the UV light wasblocked by the printed portion (masking portion), crosslinking of theopen portion on the inside enclosed by the printed portion (maskingportion) proceeded and secondary-cured, thus, the gel fraction (a) ofthe position in contact with the stepped portion after laminating becamesmaller than the gel fraction (b) at the position in contact with theflat surface portion.

Consequently, it was found that both sufficient stress relief atlaminating and reliability after laminating could be reconciled in thismanner.

In contrast, since the gel fraction at the initial stage was low, theadhesive sheet 5 was excellent in flexibility and excellent in printedstep embedding ability; however, without being able to suppress theoutgas when the laminate was heated due to being a adhesive sheet withno difference in the gel fraction between the under-print-step portionand the under-flat-surface portion, the bubbling resistance was poor.

In addition, due to being a adhesive sheet with no primary crosslinkingand the gel fraction (a) of the under-print-step portion being 0, theadhesive sheet 6 had poor cut-processability, and, readily causingpermanent deformation during storage, had poor stability.

KEYS TO THE FIGURES

-   1 constitutive member for display device-   2 adhesive sheet-   (1) print-stepped portion-   (2) print-stepped portion-   (3) flat surface portion-   3 constitutive member for display device

The invention claimed is:
 1. An image display device, comprising aconstitutive member that comprises, on a laminating surface, a steppedportion and a flat surface portion excluding the stepped portion,wherein the image display device further comprises a UV cured productand a second constitutive member, the UV cured product is filled inbetween the constitutive member and the second constitutive member, theUV cured product is formed by a non-solvent series adhesive compositionwhich does not contain a solvent, a thickness of a maximum thicknessportion of the UV cured product is 250 μm or less, the UV cured productat a position in contact with the stepped portion has a gel fraction (a)thereof of 10% or greater and smaller than a gel fraction (b) at aposition in contact with a flat surface portion, and the difference[(b)−(a)] between the gel fraction (a) of the UV cured product at aposition in contact with the stepped portion and the gel fraction (b) ofthe UV cured product at a position in contact with the flat surfaceportion is from 5 to 30%.
 2. The image display device according to claim1, wherein the gel fraction (a) of the UV cured product a position incontact with the stepped portion is from 30 to 80% and the gel fraction(b) of the UV cured product at a position in contact with the flatsurface portion is from 35 to 100%.
 3. The image display deviceaccording to claim 1, wherein the UV cured product comprises a(meth)acrylic acid ester copolymer.
 4. The image display deviceaccording claim 1, wherein the a UV cured product comprises a(meth)acrylic acid ester copolymer of 100,000 to 700,000 in mass averagemolecular weight and an intermolecular hydrogen-abstraction typephotoinitiator.
 5. The image display device according to claim 1,wherein the constitutive member and the second constitutive membercomprise a combination of any from a group comprising a touch panel, animage display panel, and a surface protective panel having aprint-stepped portion that blocks light.